This invention pertains to radio receiver circuits and, more particularly, to a circuit for demodulating one or more phase modulated digital signals.
A conventional selective call radio paging system includes a base station transmitter and a plurality of portable selective call receivers, commonly called "pagers". The pagers are usually attached to an article of clothing worn by the user, and each pager includes a unique preprogrammed address code. The base station is typically connected to the public telephone system. To send a message to a particular pager, a caller dials the base station and, after indicating which pager is to receive the message, transmits the message to the base station over the telephone system. This message is typically a voice or digital message, such as the caller's telephone number. The base station then transmits the unique address code of the intended pager, followed by the message. Although all pagers in the system can receive the transmitted message, the message will be conveyed only to the user whose pager has the address code that matches the one transmitted by the base station. The message is typically conveyed to the user by way of a speaker or digital display.
The disadvantage of such a conventional paging system is that the user has no way to communicate back to the caller. In U.S. application Ser. No. 141,654, filed Jan. 7, 1988, however, an "acknowledge-back" or "ack-back" paging system is described in which brief communications from the pager to the base station and caller can occur.
The operation of such an ack-back paging system is illustrated in FIGS. 1 and 2. FIG. 1 is a timing diagram that illustrates how the "outbound" address and message are transmitted from the base station to the pager, and how the pagers transmit their individual "inbound" acknowledgement messages back to the base station. Referring to this figure, the base station first accumulates a plurality of "N" messages and corresponding addresses (N is typically 16 or 20). When N messages have been accumulated, the addresses that correspond to each of the N messages are sequentially transmitted, followed by the sequential transmission of the N messages. (A brief reference carrier burst is transmitted between the address and message blocks.) A predetermined time after the last outbound message is transmitted, all N pagers transmit acknowledgement messages back to the base station simultaneously, but on different frequency subbands. These subbands are illustrated in FIG. 2. Referring to this figure, the outbound addresses and messages, which are sequentially transmitted from the base station, occupy nearly the entire bandwidth of the channel. The inbound acknowledgement messages, which are simultaneously transmitted from each of the N pagers, are each transmitted on a separate frequency subband. For example, the pager that receives the first outbound message from the base station would transmit an inbound or "ackback message" on subband 1, the pager receiving the second outbound message would transmit an ack-back message on subband 2, etc., etc.
The problem introduced by such an unconventional paging system is that the base station receiver must simultaneously receive and demodulate N (again, N is typically 16 or 20) ack-back messages. Although the N ackback messages are transmitted "simultaneously", they are not precisely synchronized to one another. To receive and demodulate these N ack-back messages, a plurality of N asynchronous circuits could be used in the base station receiver. Some form of synchronous demodulation, however, would be preferred because of its theoretical superior performance in low signal to noise ratio environments. But the ack-back messages are typically very short, on the order of 7 or 10 bits. Thus, a conventional synchronous circuit that utilizes a phase locked loop (PLL) would be unsuitable to demodulate these brief ack-back messages because conventional PLL circuits require a predetermined period of time within which to acquire phase lock.
Accordingly, the invention described below provides for the pseudo-synchronous demodulation of N messages that may be simultaneously transmitted on different frequency subbands.